1. Field of the Invention
The present invention relates to a high speed tool steel which in its quenched state has much higher carbides content than the conventional high speed tool steel, has excellent wear resistance, and toughness which results from uniform and fine dispersion of said carbides in matrix and has a high cutting endurance.
2. Description of the Prior Art
A high speed tool steel generally contains a large amount of elements which form carbides such as tungsten, molybdenum, vanadium, chromium, etc., most of which form carbides and are not completely dissolved in the matrix even by quenching and remain in the steel to impart abrasion resistance.
For example, the high speed tool steels which are generally used at present contain about 15- 30 % (by weight) of carbides in their annealed state, but in their quenched state a part of these carbides are dissolved in the matrix and about 5- 16 % of the carbides remain and greatly contribute to improvement in wear resistance. The wear resistance of high speed tool steels which have a sufficiently hardened matrix depends on amount, kind, size, distribution of said remaining carbides, among which kind and amount of the remaining carbides have the greatest effect on the abrasion resistance.
Kind and amount of the remaining carbides are determined by the amounts of tungsten, molybdenum, vanadium and carbon added to the steels. With increase in the amounts of tungsten, molybdenum and carbon added, amount of M.sub.6 C type carbides containing tungsten and molybdenum increases and with increase in the amounts of vanadium and carbon, amount of MC type carbides containing a large amount of vanadium increases. With increase in the amount of carbides remaining in the steel, the wear resistance increases. In this case, increase in the amount of MC type carbides which are harder than M.sub.6 C type carbides is more effective for improvement in the wear resistance and in general, amount of vanadium and the corresponding amount of carbon are increased for improving wear resistance of a high speed tool steel. However, in the conventional melting process, increase in the amounts of vanadium and carbon results in coarsening of the MC type carbides to cause reduction in hot workability and toughness and to make grinding-working impossible. For these reasons, such steels cannot be made into practical tools and amount of vanadium added has been limited to less than about 6 % except some particular cases. With reference to increase in the amounts of tungsten and molybdenum, when tungsten equivalent exceeds 20 %, ferrite remains to cause reduction in hardness and according to the conventional melting process, coarse dendrite network of M.sub.6 C type carbides are precipitated in cast state to markedly deteriorate hot workability and toughness. Therefore, tungsten equivalent for practical steels is limited to less than 20 %. The term "tungsten equivalent" means the sum of amount of tungsten in % by weight and twice the amount of molybdenum in % by weight. As is easily understandable for one skilled in the art, this is the value of amount of molybdenum added to a high speed tool steel which is indicated by the amount of tungsten. However there are some examples which contain more carbides former elements than conventional high speed tool steels for the purpose of improving wear resistance. For example, Japanese Patent Publication No. 21534/69 discloses steels of SKH 9 class in which the vanadium content is increased to 12.2 % and those of SKH 55 class in which the vanadium content is increased to 10 %.
However, all of these known examples aim at improving the wear resistance by increasing the vanadium content in the steel to increase the amount of crystal of MC carbides and in these known examples the tungsten equivalent in the steel is not increased to at least 24 % as in the present steel.
Thus, in the conventional melting process, there are limitations in the amounts of tungsten, molybdenum and vanadium and amount of the remaining carbides at quenching is limited to about less than 15 %, which is the definite reason for the fact that such steels are inferior to carbide tools in wear resistance.